This invention relates to a communication system, and more particularly relates to a communication system in which a data terminal adapted to an analog communication line is connected to a digital communication system.
In a data terminal, such as a facsimile apparatus, which needs to identify, to supervise and to control data transmission according to a designated protocol, data transmission is performed using not only a main channel but also a sub-channel. The latter is used for transmission of code data i.e., a hand shake protocol, whereas the former is used for transmission of primary data.
Analog communication systems have been dominant for public and leased lines, and have been developed mainly for telephone communications. Data terminals or data transmitting apparatus have also been developed and adapted for analog transmission even though digital data process techniques are fully used for their circuitry.
More developments will be made in digital communications systems in the future and such systems will play a greater role in the communication of information. However, for the time being, communication systems will continue to be used with data terminals adapted to analog communication lines and which have partially been adapted to digital communication systems by means of a suitable adapter. In fact, there is a great demand to adapt analog line data terminals to data terminals which can be used in a mixed communication system. This is so mainly because digital communication lines are cheaper and have a higher quality than analog communication lines. Moreover, data terminals for digital lines are not as popular as those for analog lines.
There have been several attempts in the prior art to adapt facsimile apparatus for use with analog communication lines so that they can be used with digital communication lines. FIG. 1 shows one such facsimile communication system. In FIG. 1, two digital terminal stations 1 and 2 are connected to each other by digital communication line 3. Two facsimile apparatuses 4 and 5 are connected to respective digital terminal stations by analog communication lines 6 and 7. Facsimile apparatus 4 and 5 are apparatus for facsimile transmission over a conventional switch telephone network which use a band-width between 0.3 kHz and 3.4 kHz. Such networks are capable of transmitting facsimile message data at a signaling rate of of 9600 bps and binary coded information at a signalling rate of 300 bps for example. Digital terminal stations 1 and 2 operate to change the signalling method for call establishment and call release between the analog communication line and the digital communication line. They also operate to convert analog signals to digital signals by code modulation and vice versa. The length of analog communication lines 6 and 7 is relatively short and the length of digital communication line 3 is relatively long. In this system, after call establishment facsimile transmission is performed in real time. In a facsimile call, one facsimile apparatus can transmit facsimile message data to the other at a data signalling rate of 9600 bps, each exchanging binary coded information at 300 bps.
In this way, the facsimile apparatus for an analog communication line is able to be adapted to a digital communication line. However, in such a system, it is difficult to make much use of the benefit of low communication line cost because a high data signalling rate is required. A data signalling rate of 64K bps is required to modulate a 3.4K Hz analog signal in PCM, in the case where the sampling frequency is 4K Hz and the number of quantum steps is 128. If ADMCM (Adaptive Differential Pulse Code Modulation) is utilized for the above modulation, the data signalling rate can be reduced. However, it can only be reduced to 32K bps at a minimum.
FIG. 2 shows another conventional facsimile communication system. In this system, message storing apparatus 8 and 9 are provided within digital terminal stations 1 and 2. Message storing apparatus 8 and 9 correspond to a facsimile apparatus from which the scanning unit, the printing unit and the associated components are eliminated and to which a memory of large-sized capacity is added. Facsimile transmissions between facsimile apparatus 4 and facsimile apparatus 5 is performed through three transmission calls. First, facsimile apparatus 4 originates a transmission call (first call) into analog communication line 6 designating facsimile apparatus 5. Message storing apparatus 8 then operates as a facsimile receiver and receives the facsimile message data from facsimile apparatus 4 and an exchange of binary coded information between the two takes place. Facsimile apparatus 4 then sends its facsimile message data to message storing apparatus 8. Message storing apparatus 8 then stores the received message data and an address signal transmitted from facsimile apparatus 4 so as to designate facsimile apparatus 5. However, message storage apparatus 8 does not store binary coded information exchanged during its receiving operation. Next, digital terminal station 1 originates a transmission call (second call) to digital terminal station 2 to transfer the formerly received message data and the address signal. In this transmission call, it is only necessary that the data be transferred in the manner of a so called "file transfer." Therefore, there is no need to produce facsimile binary coded information in this second transmission call through the digital communication line. The transferred message data and the address data are stored in message storing apparatus 9. The data signalling rate is free from that of the facsimile message transmission. It is only dependent on the rate of the digital communication line. After completion of the second call, message storing apparatus 9 originates a transmission call (third call) to facsimile apparatus 5 according to the received address signal. Facsimile apparatus 5 thus operates as a facsimile transmitter. Facsimile apparatus 5 forms the required binary coded information to communicate with facsimile apparatus 5 then transmits the message data to facsimile apparatus 5 at the data signalling rate of 9600 bps.
As described above, it is possible to connect a data terminal adapted for an analog communication line to a digital communication system by use of a storage apparatus. As mentioned above, however, a plurality of transmission calls must be made for one transmission call from one end to the other end. Thus, real time operation is not possible and a user is not able to confirm completion of a data transmission to the ultimate designation. Moreover, such a system requires sophisticated digital processing equipment and a memory having a large capacity for storage of message data. Such systems are required to process data transmission procedure almost the same as that of a data terminal.